In general, a tire vulcanization mold is firstly made by casting, and then is finished by subjecting its surface to an abrasive blasting process using abrasive grains made of alumina or the like.
On the surface of the mold that has been subjected to an abrasive blasting process, irregularities of random diameters and depths are formed. Once these random irregularities are transferred to the surface of the vulcanized tire, the tire surface disorderly diffuses light, and thereby the tire ends up having no gloss while having a black and yet coarse texture.
Herein, the term “no gloss” refers to the condition in which the value measured by a gloss meter is in the range of 0.1 to less than 1. A tire having no gloss often has a coarse texture on its surface.
In this regard, for example, Patent Document 1 (JP 2000-142026 A) discloses a pneumatic tire wherein the surface roughness (RaT) of the molded tire is in the range of 1.5-20 μm when it is measured by a stylus-type surface roughness tester. Patent Document 1 describes that the surface gloss of the molded tire may be enhanced and the appearance of the tire may be improved.
Patent Document 2 (JP 2004-017964 A) discloses a tire mold having a radially inner surface for forming a tread and a pair of sidewalls, the tire mold comprising: at least one sidewall forming surface having a surface finish of less than 0.381 microns. There is described that a highly light reflective surface on a tire sidewall is achieved in a novel way. The resultant tire has highly light reflective surfaces achieved in a way that is clearly recognizable regardless of the observer's relative position.
The art disclosed in aforementioned Patent Documents 1 and 2 enhances the gloss of the tire surface and enhances the reflectance of the tire surface, by controlling the surface roughness Ra, thereby improving the appearance of the tire.
The inventors manufactured tires using two types of molds that were subjected to different surface processes and examined the surface roughness Ra and gloss of the tires. They found out that the tires with substantially equal surface roughness Ra did not always have the same measurement gloss value. The evaluation results are shown in Table 1.
In Table 1, Process A indicates an abrasive blasting process performed on a surface of a tire mold using alumina abrasive grains, and Process B indicates an abrasive blasting process performed on a surface of a tire mold using spherical abrasive grains.
TABLE 1Processing methodProcess AProcess BSurface roughness Ra3.72 μm3.42 μmTire gloss0.42.7
According to Table 1, out of two types of surface processing methods, i.e. Process A and Process B, by which tires of substantially equal surface roughness Ra are obtained, Process B achieves the tire gloss of 2.7, which is good, while Process A achieves the tire gloss of 0.4, which is insufficient.
According to the foregoing results, in order to control the tire gloss so as to constantly achieve a tire with the gloss of not less than 1, it is not sufficient to merely maintain the surface roughness Ra of the tire at a constant value.
Patent Document 3 (JP 2003-300214 A) discloses a tire vulcanizing mold having a molding surface for vulcanizing a tire, wherein at least a part of the molding surface is provided with a rough-surface molding surface, which is configured such that its ten points mean roughness Rz is 5-100 μm and it has the surface roughness such that its average distance between peaks in a local area S is 20-150 μm. Patent Document 3 describes that, since the mold has the rough-surface molding surface which has limited surface roughness, it is possible to form a pneumatic tire comprising, on its outer surface, a rough-surface portion having substantially the same surface roughness as the rough-surface molding surface. Such a rough-surface portion can diffusely reflect extraneous light to a preferable extent, thereby enabling the tire to appear deeply black. Accordingly, additives seeping through the tire etc. may be less visible and the appearance of the tire may be improved over a long period of time.
The inventors manufactured tires using the tire vulcanizing mold having the structure of the invention disclosed in aforementioned Patent Document 3 and evaluated the surface condition and tire gloss of the tires. They found out that, when such a tire vulcanizing mold was used, tires having no gloss while having a black and yet coarse texture were sometimes manufactured. The evaluation results are shown in Table 2,
TABLE 2Ten points mean roughness Rz of the mold13.1 μmAverage distance between peaks in a 48.4 μmlocal area S of the moldCondition of the tire surfaceblack and yet coarse surface textureTire gloss0.4
According to the results shown in Table 2, the tire gloss cannot be sufficiently controlled by adjusting parameters such as the ten points mean roughness Rz and the average distance between peaks in a local area S of the mold, and thereby the tires sometimes end up having an insufficient gloss while having a black and yet coarse texture.
Accordingly, it may be concluded that there are other factors than the aforementioned parameters that cause inconsistency in the tire gloss, and it is impossible to constantly obtain tires with an appropriate gloss without controlling these factors.
On the contrary, if the gloss of the tire surface is too high, in other words if the gloss of the tire surface is not less than 6.0, the tire ends up having a whitish, shiny appearance, which is not preferable.
Further, even if the gloss is appropriate, as discussed above, the tire surface may have a coarse texture, in which case the appearance of the tire is not preferable. In this regard, it may be concluded that the factor lies in the parameters for roughness of the tire surface.
In general, a preferable appearance of a tire is achieved when the tire surface has an appropriate gloss and has a deeply-black and moist texture. However, as discussed above, it is impossible to constantly supply such a tire using the prior art disclosed in Patent Documents 1-3.
Therefore, in order to constantly supply tires with a good appearance, it is required to contemplate a method of manufacturing a tire vulcanization mold in which roughness parameters for the tire surface that determine the gloss and texture of the tire surface are adjusted and in which such roughness parameters may be appropriately transferred to the tire.